The Genetic Modification Core will generate genetically engineered human ES (hES) cell lines required for the four primary projects and the pilot projects of the Program. These include gene null, conditional null, and knock-in hES cell lines. In addition, a very efficient recombination medicated cassette exchange method will be used to generate gene overexpression and/or knockdown (by RNAi) hES cell lines. Gene constructs generated by the Program investigators will be electroporated into NIH-approved human ES cells (H1 or H9) and cultured with appropriate drugs to select for DNA transformants. The resulting hES cell colonies will be picked and expanded for genotype analysis. Genotype analysis will be performed by the Project investigators. After the genetically engineered hES cell lines are identified and confirmed they will be transferred to Core B for karyotype analysis, differentiation potential (if required) in vitro and in vivo by teratoma formation, and expansion for cell banking. All of the four primary projects propose to generate genetically modified hES cell lines. Thus, the Genetic Modification Core is an essential component of the Program.